1. Technical Field
The present invention relates to electroless plating, and more particularly to Co-based electroless plating for semiconductor devices.
2. Description Of The Related Art
On-chip magnetic inductors or transformers are passive elements that find wide applications in on-chip power converters and radio-frequency integrated circuits. On-chip magnetic inductors or transformers are composed of a set of conductors (e.g., copper lines) to carry the currents and a magnetic yoke/core to store magnetic energy.
High performance magnetic core materials often determine the performance of the inductors both in inductance (L) and quality factor (Q), especially in the high frequency range (>10 MHz). The figures of merit for the soft magnetic materials used for on-chip inductors are high permeability, high moment, low coercivity, high anisotropy, and high electrical resistivity.
Co-based amorphous alloys such as Co—Zr—Ta, Co—Zr—Nb and similar systems are particularly of note. These Co-based amorphous alloys have superior magnetic properties and relative high electrical resistivity. The amorphization of the cobalt is achieved by introducing small amounts of glass forming elements, mostly non-magnetic transition metals (e.g., Ti, Zr, Nb, Ta, W and so on). These elements can also be used to tune the magnetic properties, e.g., magnetostriction. On-chip inductors incorporated with such materials show favorable high-frequency response. However, these Co-based amorphous alloys are deposited mostly by vacuum deposition techniques (e.g. sputtering). This is because most transition metals are too noble to be reduced electrochemically in an aqueous solution.
Vacuum methods usually have low deposition rates, generally do not have good conformal coverage and the derived magnetic films are difficult to pattern subtractively due to the challenges of mask alignment and long etching times. In addition, such thick film sputtering requires high vacuum and frequent system maintenance due to collateral coating of chamber walls, which makes sputter processing expensive and hinders practical integration and manufacture of these materials. The deposition processes can result in high stress on a wafer, and, in particular, on large scale wafers (e.g., >200 mm).